The art of dispensing liquids onto a rotatable substrate to form useful coatings is well known.
U.S. Pat. No. 3,695,911 teaches both a method and an apparatus for applying materials such as photoresist onto the surface of a spinning slice of semiconductor material.
U.S. Pat. No. 4,267,208 teaches the coating of a lens which is dipped into a pool of a vinyl copolymer, withdrawn from the pool, and spun to provide a uniform protective coating thereon.
U.S. Pat. No. 4,378,953 teaches the formation of a thin membrane by dispensing a polymer onto a spinning substrate to form a layer of the polymer on the substrate.
U.S. Pat. No. 4,451,507 teaches that a uniformly thick layer of a slurry of glass can be formed on the surface of a semiconductor material by rotating the semiconductor material as the glass slurry is dispensed thereon as a spiral bead.
IBM Technical Disclosure Bulletin, Vol. 17, #11, April 1975 on pages 3211-3212 discloses a typical photoresist dispensing apparatus presently in use in the Semiconductor Industry.
An article entitled, "Thickness Variance of Spun-On Photoresist, Revisited" appeared in the Proceedings of the Kodak Microelectronics Seminar, 1978, and teaches that thin layers of photoresist material may be spun on silicon slices.
Liquid photoresists are, today, applied to semiconductor wafers from the top as shown in the IBM Technical Disclosure Bulletin, Vol. 17, #11, April 1975, pages, 3211-3212.
This method causes a puddle of photoresist to be dripped onto the face of the wafer being coated, after which the wafer is spun at a high rotational speed. The centrifugal forces of the spinning overcomes the surface tension of the puddle of photoresist and results in a thin film of photoresist on the surface of the wafer.
This technique although widely and commonly used has several drawbacks. For example:
When levels of topography increases, the actual film thickness of the coating increases because of the effects of surface tension, gravity, substrate material properties, viscosity, spin speed, temperature, and etc. Moreover contaminants on the surface such as dust particles and the like can become trapped in pockets, trenches or the like.
Also, to avoid voids, pinholes, and assure coverage, excess amounts are puddled on the surface to be coated. Of the amount so puddled about 99% is spun off and thus wasted. For example, on a four inch wafer 1.5 to 2.0 ml of photoresist is usually used. This amount if allowed to remain on the surface would produce a uniform depth of at least 0.007 inches. In actual fact after spinning an average depth of 0.00005 inches remains. Thus at least 140 times more photoresist is puddled on the wafer than is actually needed to coat the wafer to the desired thickness. All of this excess photoresist is lost. Since photoresist costs about $600.00 per pint one can readily see that a large part of this cost is lost.
Still further there is the problem of backside contamination where unwanted photoresist is deposited on the wafer on its reverse face. This photoresist may be deposited on the back of the wafer where it is not wanted because of air turbulence, back splatter from the walls of the surrounding bowl or from extra drips that fall on the wafer holding chuck. This backside contamination can cause serious focusing problems in photolithographic tools.